Light emitting diodes (LEDs) are replacing other light sources in applications such as handheld flashlights, backlights for LCD displays, camera flash units, etc., due to efficiency, reliability, cost, and other factors. LEDs, however, have specific drive requirements that must be observed for proper operation. For example, the amount of light emitted by an LED is determined primarily by the amount of drive current applied to the device. Human vision is highly sensitive to light fluctuations caused by small variations in the drive current, and therefore, accurate currents sources must be used to drive LEDs. With relatively large display panels, multiple LEDs or strings of LEDs must be used to backlight the entire display. Thus, not only must the current sources have a high level of absolute accuracy, but they must also be well-matched relative to each other to prevent some regions of the display from appearing brighter than other regions.
The development of white LEDs has contributed to the increasing popularity of LED light sources, but has also increased the demands on the circuitry used to drive these devices. For example, the operating voltage for a white LED may be several times greater than the operating voltage of a red LED. Therefore, a boost converter may be required to boost the voltage of a battery in a portable device to an adequate level for operating a white LED. Thus, drive circuitry for LED lights sources has become increasingly complex and created more opportunities for harmful fault conditions. Even in applications where a boosted power supply is not required, fault conditions have become more problematic.